This invention relates generally to particle analysis instrumentation and techniques, and more particularly concerns apparatus and method for collection of particle samples in such manner as to facilitate analysis in a scanning electron microscope (SEM), eliminating handling between sampler and SEM.
There is a continuing need to collect and identify small particles which, for example, are dispersed or entrained in gases, such as air. By way of illustration, manufacturers of semiconductors, pharmaceuticals, and other high precision products are very concerned about the presence of airborne microcontaminants in their manufacturing facilites. The presence of airborne particulate contaminants can cause serious drops in production yields and increase product failures. They therefore invest heavily in clean rooms to provide highly filtered air and a clean environment for their manufacturing plants. Very small respirable airborne microcontaminants are also of serious concern to researchers in environmental health.
However, because of filter failures, or the presence of personnel, or for a number of other reasons, the concentration of airborne particles can or does increase to unacceptable levels. When this happens, it is very important to know what the particles are and where they came from.
At the present time, the routine monitoring of the presence of airborne particles is carried out through the use of optical particle counters. This type of instrument utilizes light scattering principles to report the number of particles present per unit volume, for particles larger than 0.3 microns. It does not retain the samples being measured; they simply pass through the instruments.
To find out what the particles are requires the collection of samples of the particles and then their identification through modern microanalysis techniques, i.e., Scanning Electron Microscopy (SEM) and X-ray Energy Spectroscopy (XES), such as Energy Dispensive X-ray Analysis (EDXRA). Because the particles are as small as 0.1 microns or less, the high magnification capabilities of the scanning electron microscope are required. Optical microscopes are not good enough.
The high magnification and three dimensional capabilities of the scanning electron microscope provide detailed information on the shape, size, texture, topology, and the structure of individual particles. An electron microscope with an X-ray detector attachment gives it the added capability of EDXRA to provide elemental composition information on the particle sample as well. SEM and EDXRA are established techniques for microanalysis of very small particles.
In an electron microscope an electron beam performs the function of a light beam in an optical microscope. When a particle specimen is being examined, it is the electron beam that bombards the particle. If the particle is nonconductive, a charge build-up on the particle caused by the electron beam bombardment must be minimized. To accomplish this, the sample is coated with a thin layer of carbon which is conductive, or with gold.
Accordingly, electron microscopists think in terms of physically transferring particles from non-conductive substrates and mounting them on conductive substrates which are integral parts of conductive holders which can fit directly into a scanning electron microscope (S.E.M.). If, for example, the particles are collected on a filter, the microscopist must either cut a piece of the filter paper with particle sample on it and mount it on the conductive holder substrate, and then coat it with carbon, etc., or he must remove and transfer the particle specimens individually.
All these procedures are very time consuming and fraught with errors and the introduction of contamination. For example, if one can mount a clean 1 .mu.m (10.sup.-12 g) particle on a polished beryllium plate, one can look at it with the scanning electron microscope and analyze it quantitatively with the electron or ion microprobe. The transmission electron microscope will provide a good look at 10.sup.-15 g (0.1 .mu.m.sup.3) particles, as well as a good electron diffraction pattern. The ion microprobe will detect 10.sup.-18 g of any element in femtogram (10.sup.-15 g) particles.
Most samples can be examined directly without fractionation. If, however, the particle of interest is embedded in a matrix and if it must be removed for identification either microscopically or by other techniques, another major problem is introduced. The particles must be isolated, or separated into distinct groups, at least in the field of view of the microscope. Often they must be actually picked out physically.
Manipulation of tiny single particles is not easy. It requires a great deal of skill on the part of the microscopist and fine tools. A careful microscopist with very steady hands can, however, with some practice, "pick up and deliver" particles smaller than one micrometer in diameter. The modus operandi for a particular particle depends on where it is, that is, on the medium that surrounds or supports it. In general, it may be lying on or embedded in paper, glass, metal, ceramic, paint, polymer film or any other material. Or it may be mixed in a liquid medium (water, glycerin, etc.) on a microscope slide, in some other refractive index medium or in the adhesive layer of transparent tape. In any case, the microscopist must spend time and exercise great care in extraction the particle samples for mounting on a conducting substrate and mount before analysis in an S.E.M.
Very few experts in X-ray diffraction, electron microscopy or microprobe analysis are adept at handling single particles near the limits of sensitivity of their instruments. The techniques require a clean atmosphere, steady hands, practice and very fine needles. Such procedures are time consuming and require adherence to the following:
(1) never take one's eyes off the particle during a manupulative operation; PA1 (2) remove the particle from the needle only by "washing" it off into a micro drop of liquid to which the particle will adhere or in which the adhesive holding the particle to the needle will dissolve; PA1 (3) to be picked up, a particle must be loose, be touched with a liquied film on a needle more viscous and tacky than its environment, or be picked using a freezing needle to pick up a small frozen volume of the liquid film with the occluded particle. Excess liquid around a particle is removed in different ways, depending on the fluidity. If it is quite fluid, the excess liquid can usually be soaked into a small triangle of thin lens tissue while one keeps a close watch on the particle with the microscope. If the liquid is very viscous (e.g. Canada balsam or Aroclor), the same operation is carried out, but after dilution with benzene. Because each situation is unique, patience, ingenuity and self-confidence are the essential ingredients for success. PA1 (x) a holder mounted, or to be mounted, in the cavity in association with said structure and having an electrically conductive surface integral to or mounted on a conductive stem facing toward said orifice or orifices in the path of flow from the orifice and normal to it so that particulates collect on the holder proximate said surface, PA1 (y) the holder sized and adapted for use in the scanning electron microscope, whereby the holder may be directly transferred to said microscope for electron beam scanning of said particulates. PA1 (i) flowing said gas and entrained particulates through said nozzle and toward said holder surface, PA1 (ii) collecting particualtes on said surface, PA1 (iii) then demounting said holder from said structure and directly transferring the holder to a scanning electron microscope, and PA1 (iv) operating the microscope to scan the particulates on the holder surface.
From the foregoing, it is clear that a need exists for simplification of particle collecting and handling techniques, particularly as respects collected particle transfer to an SEM.